The exterior angles are made by extending the lines of the three sides. The angles that they form outside of the triangle are what we call the exterior triangles. However, since they are vertical angles, they are still congruent with their corresponding interior angles.
The total sum of interior angles of a triangle is 180 degrees. So,
180 = x + 2x + 3x
180 = 6x
x = 180/6
x = 30
The answer is C.
